Printed circuit substrate with controlled placement covercoat layer

ABSTRACT

A covercoated base substrate, comprising a base substrate and a photoimageable covercoat layer having a lead portion extending beyond at least one, wherein the lead portion has a controlled offset distance from the edge, and a printed circuit made from such base substrate having at least one conductor including a trace and a lead wherein the covercoat has a lead portion covering at least one lead. A base substrate may also have an opening formed therethrough, the opening defining an interior edge wherein the covercoat extends beyond that interior edge.

This is a divisional of application Ser. No. 09/378,080 filed Aug. 20,1999, now pending.

BACKGROUND

The disclosures herein relate generally to flexible circuits and moreparticularly to flexible circuits with covercoat layers formed to have acontrolled placement.

Flexible circuits generally include a pattern of conductive traces thatare supported on a base substrate such as a layer of dielectricmaterial. The conductive traces typically have a copper core plated witha corrosion resistant material such as gold. Polyimide is a common basesubstrate. U.S. Pat. Nos. 4,987,100; 5,227,008; 5,334,487; 5,557,844 and5,680,701 disclose processes for fabricating printed circuits having aflexible polymeric base substrate such as polyimide. U.S. Pat. Nos.3,660,726; 4,029,845; 4,526,835; and 5,806,177 disclose processes forfabricating printed circuits having a generally rigid base substratesuch as a glass reinforced epoxy composite.

Electronic packages, medical devices, hard disk drive suspensions andink jet printer pens are common applications for flexible circuits.Flexible circuits offer attributes such as fine pitch traces, complexcircuit designs and flexibility. Depending on the design and specificapplication, a flexible circuit may have an opening formed in the basesubstrate. One or more of the conductive traces may include a lead thatextends in a cantilevered manner from an edge of the opening. The leadsmay also be formed in a manner in which they the span across theopening.

In some applications, flexible circuits may be exposed to an aggressiveenvironment. Unprotected conductive traces and the interface between theconductive traces and the base substrate are two areas susceptible tobeing affected by adverse environmental conditions such as exposure tocorrosive fluids. Exposing unprotected conductive traces to adverseenvironmental conditions typically leads to the traces corroding ordelaminating from the base substrate.

The flexible circuit is typically attached to a rigid structure such asa stiffening member or the body of a printer pen. The leads may beinterconnected to an electronic device carried by the rigid structure orto an electronic device that is attached directly to the base substrateof the flexible circuit. Typically, the side of the base substratecarrying the conductive traces is attached to the rigid structure. Anencapsulant is typically applied over the leads to provide a degree ofprotection from adverse environmental conditions.

A covercoat layer is sometimes formed over the conductive traces toprevent the traces from being exposed to adverse environmentalconditions. The covercoat layer is often a photoimageable material thatis patterned using UV light and a photomask. Due to limitations inconventional methods of forming the covercoat layer, the resultingcovercoat layer does not have a uniform and controlled thickness or awell-defined pattern. In some areas, the thickness of the covercoat canbe insufficient to provided adequate protection against adverseenvironmental conditions. This is often the case with circuits havingportions of the traces beyond an edge of the substrate (called leads).

An encapsulant is often applied to protect the leads. Depending on thetype of device attached to the leads, the encapsulant may also be usedto protect the device (e.g. a bare semiconductor chip). Because of theorientation of the flexible circuit, it is easy to encapsulate theoutward facing side of the conductive traces. However, reliablyencapsulating the inward facing side of the circuit is difficult. Airpockets formed adjacent to the leads during encapsulation can serve aspathways for contaminants to reach the traces. Over time, traces havingan insufficient thickness of covercoat material can be attacked bycontaminants, causing the flexible circuit, the attached device, orboth, to fail.

Accordingly, a need has arisen for a base substrate having a covercoatformed thereon in which the shortcomings of previous techniques andconstructions are overcome.

It has now been discovered that a photoimageable covercoat can be formedwhich has controlled placement beyond the base substrate and onto thelead portion of the conductors. Use of a covercoat with such improvedplacement characteristics also helps to alleviate problems caused by airpockets in the encapsulant by providing an additional layer ofprotection. Further, a photoimageable covercoat can be formed which canbe coated to a controlled thickness beyond the edge of the basesubstrate to assist in corrosion protection, and the like.

SUMMARY

The invention provides a printed circuit having improved resistance toadverse environmental conditions. The printed circuit includes a basesubstrate having conductors formed thereon. A portion of some or all ofthe conductors form traces and a portion form leads extending from anedge of the base substrate. A photoimageable covercoat layer is formedon the base substrate including a lead portion formed on the leadsextending beyond the substrate at one or more edges. The lead portion ofthe photoimageable covercoat layer has a controlled and substantiallyuniform placement, i.e., a predetermined and substantially controlledoffset distance.

The lead portion of the photoimageable covercoat may be a directextension of a trace portion of the photoimageable covercoat, i.e., itmay be a continuous coating, or it may be a completely detached coating.

The base substrate and printed circuits formed therefrom also have anonconductive portion, i.e., that part of the substrate which isuncovered by a conductor. Some or all of this portion may also becovered by the covercoat.

n a preferred embodiment, the photoimageable covercoat also has acontrolled thickness beyond the edge of the base substrate, and coversat least a portion of the upper surface and the lower surface of theleads.

A principal advantage of the embodiments presented herein is that theleads can be encapsulated with a uniform layer of covercoat material toreduce the potential of the leads being attacked by contaminants. Auniform layer of covercoat can be formed on the leads to encapsulate andprotect the leads.

Another principal advantage is the ability, when using a substratehaving an interior opening, to form three-dimensional features withinthe opening on the lead portion of the covercoat; a conductor having alead extending into such opening need not be present to form such afeature.

As used herein, these terms have the following meanings:

1. The term “trace” refers to that portion of the conductor(s) which issupported on a base substrate.

2. The term “lead” refers to that portion of the conductor(s) which isunsupported by the base substrate, e.g., a conductor extending beyond anedge of the polyimide substrate.

3. The term “lead portion” when referring to the photoimageablecovercoat refers to the portion of the covercoat which extends beyond anedge of the polyimide substrate. Note: this term is used for the portionof the covercoat extending beyond an edge of the substrate whether ornot a conductor lead also extends therefrom.

4. The term “trace portion”, when referring to the photoimageablecovercoat refers to the portion of the covercoat which coats at leastone trace portion of the conductor on the base substrate.

5. The term “UV” means ultraviolet and refers to radiation from a sourcehaving wavelengths of from about 100 to about 4500 Angstroms.

6. The term “offset distance” means the distance which thephotoimageable covercoat extends beyond the edge of the base substrate.

7. The term “line and space pattern” means a photomask is used thatallows only certain portions of the covercoat surface to be exposed toUV, while other areas receive no UV exposure.

8. The term “flood” or “flood expose” refers to a UV exposure in whichat least one entire covercoat surface is exposed to UV.

9. The term “interior edge” refers to an edge originally facing anopening formed in the interior of a base substrate during processing.Such an edge may become an exterior edge on the final circuit aftersingulation.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a perspective view illustrating an embodiment of a basesubstrate with a photoimageable covercoat layer formed in an opening inthe base substrate.

FIGS. 2A-2D are cross sectional views illustrating an embodiment of amethod of forming a photoimageable covercoat layer.

FIG. 2E is a cross sectional view taken at line 2E—2E in FIG. 1.

FIG. 3 is a flow chart illustrating an embodiment of a method of forminga photoimageable covercoat layer.

FIG. 4 is a perspective view illustrating an embodiment of a printedcircuit having a photoimageable covercoat layer formed thereon.

FIG. 5 is a cross sectional view taken at line 5—5 in FIG. 4.

FIG. 6 is a perspective view illustrating another embodiment of aprinted circuit having a photoimageable covercoat layer formed thereon.

FIG. 7 is a cross sectional view taken at line 7—7 in FIG. 6.

FIG. 8 is a perspective view illustrating an embodiment of a printedcircuit with a lead portion of the covercoat layer being spaced apartfrom a trace portion.

DETAILED DESCRIPTION

A base substrate 10, FIG. 1, includes a photoimageable covercoat layer12 (hereinafter referred to as the covercoat layer 12) formed in anopening 14. The base substrate 10 may be a flexible dielectric substratesuch as polyimide, a rigid dielectric substrate such as a glassreinforced epoxy composite material, a conductive material such asaluminum or other suitable types of substrates for an intendedapplication. The covercoat layer 12 may be formed to include one or morephotoimaged features 16 having a passage 16 a that extends through theentire thickness of the covercoat layer 12 and a channel 16 b thatextends only partially through the covercoat layer 12.

Indeed, use of the preferred process enables creation of complexthree-dimensional dielectric features within the polyimide openings. Inthis embodiment, the feature is typically formed in an opening in theinterior of the base substrate. This feature is formed by UV exposure ofdiffering patterns on the top and bottom sides of the flexible circuit.Photosensitive covercoat materials typically exhibit a limited depth ofUV cure; i.e., the cure does not penetrate to the full thickness of thefilms. As shown, these features and be channels whose depth is only aportion of the thickness of the covercoat. These features can be createdby exposing one side of the flexible circuit to a “line and space”pattern and flood exposing the other side. The depth of the channel iscontrolled by the UV dosage applied to the flood exposed side.

Many materials that are used as photoimageable covercoat layers areformulated to be crosslinked in the presence of UV light to facilitatepatterning and cured in the presence of heat to achieve enhancedresistance to adverse environmental conditions, such as corrosive fluidsor gases, biological contaminants, etc. Suitable covercoat materialsinclude photoimageable epoxy acrylates, polyimides, and the like.Commercially available materials include the product sold under thetrademark “Imageflex” by Coates Circuit Products under the part numberXV601T; “PSR-4000/AUS5” sold by Taiyo America; “NPR-80/ID431” sold byNippon Polytech Corporation; the product sold by Olin-Arch under thetrademark “Probimide”, under the series number 7500 and the product soldunder the trademark “Carapace-A” by Electra Polymers and ChemicalsAmerica under the part number EMP110.

A method of forming the preferred covercoat layer 12 on the basesubstrate 10 is illustrated in FIGS. 2A to 2E. This method is alsodepicted in the flow chart of FIG. 3. The opening 14 is formed in thebase substrate 10. The base substrate 10 has a first side 18 and asecond side 20. A first side 21 of an adhered liner 22, FIG. 2B, islaminated to the second side 20 of the base substrate 10. The adherentliner 22 may be a low-tack tape having a backing such as polyester witha layer of acrylic, mastic, rubber or other suitable adhesive formed onthe backing. A key requirement for the adherent liner 22 is that itshould not build adhesion over time so that it can be easily removedfrom the base substrate 10. Another requirement is that the adherentliner removal should exhibit substantially no adhesive transfer to thesubstrate.

The covercoat layer 12, FIG. 2C, is then formed over the first side 18of the base substrate 10 and into the cavity 11, FIG. 2B, defined by theopening 14 and the adherent liner 22. The covercoat layer may be formedby applying a layer of a liquid covercoat material using a coatingmethod such as knife coating, extrusion die coating, curtain rodcoating, screen printing, spray coating or other suitable known methodsof forming a layer of covercoat material. The covercoat layer 12 is thendried at ambient temperature or in a suitable drying apparatus such asan air convection oven. Other methods of forming a covercoat layer suchas laminating a dry film layer to the substrate are also contemplatedwithin the scope of the present disclosure.

As shown in FIG. 2C, the covercoat layer 12 includes a trace portion 13and a lead portion 15. The trace portion 13 of the covercoat layer 12 isthat portion formed on the base substrate 10. The lead portion 15 of thecovercoat layer 12 is that portion of the covercoat layer 12 formed onthe adherent liner 22. The lead portion of the covercoat layer 12 may beformed adjacent to an exterior edge 17, an interior edge 19 or bothedges.

Next, the covercoat layer 12 is photoimaged, FIG. 2D. The photoimagingstep includes exposing and developing the covercoat layer 12. Theexposure step includes exposing the covercoat layer 12 to a light source23 such as a UV lamp. The depth to which the material is crosslinkedrelative to the overall thickness of the covercoat layer 12 is generallya function of the applied exposure energy. Generally speaking, thethickness of the crosslinked material increases with increasing exposureenergy. The relationship between the thickness of the crosslinkedmaterial and the exposure energy contributes to enabling the thicknessof the lead portions 15 of the covercoat layer 12 to be formed to acontrolled and uniform thickness.

Where one or more photoimaged features 16 are desired, a first photomask24, FIG. 2D, may be positioned adjacent to the covercoat layer 12 and asecond photomask 25 may optionally be positioned adjacent the secondside 20 of the base substrate 10. The photomasks 24, 25 each includeopaque regions 28 formed in a desired pattern. The opaque regions 28preclude light from being exposed to the underlying regions of thecovercoat layer 12. Upon exposing the covercoat layer 12 to a lightsource 23, the photomasks 24, 25 produce exposed portions 12 a andunexposed portions 12 b, FIG. 2D. Commercially available equipment maybe used to expose the covercoat layer 12.

Following the covercoat layer 12 being exposed, it is subjected to adeveloping solution. When a negative-acting photoimageable material isused, the portions not exposed to the UV light will be removed duringthe developing step. When a positive-acting photoimageable material isused, the portions exposed to the UV light will be removed by thedeveloping solution. After the covercoat layer 12 is developed, it iscured by conventional methods, e.g., thermally cured at an elevatedtemperature in an air convection oven, cured using infrared radiation,of simply dried at ambient temperature although this is less preferredin manufacturing due to time constraints.

FIG. 2E shows a photoimaged feature 16 which has been formed over theinterior opening.

In a lesser preferred method for forming a covercoat layer, anon-adherent liner is used. The non-adherent liner is brought intocontact with, but not adhered to, the base substrate. The liner preventscovercoat material applied adjacent to openings in the base substrateand adjacent to the edges of the base substrate from contaminating theprocess equipment. However, because the non-adherent liner is notphysically attached to the base substrate, it is somewhat susceptible toshifting relative to the base substrate during processing, and assistsin placement of the covercoat. To prevent the non-adherent liner fromshifting and unintentionally smearing covercoat material onto adjacentareas of the base substrate, the non-adherent liner is removed prior tothe covercoat layer being dried. During the drying process, thecovercoat material adjacent to the edges of the substrate may flow intoadjacent portions of the base substrate, causing the thickness of thecovercoat layer to be non-uniform. Accordingly, the use of anon-adherent liner reduces the formation of a uniform and controlledthickness on the lead portion.

In the preferred method of forming the covercoat, attachment of theadherent liner 22 to the base substrate 10 enables the formation of acovercoat layer 12 including lead portions 15 having a relativelycontrolled and uniform placement and thickness. Because the adherentliner 22 is adhesively attached to the base substrate 10 it can remainattached to the base substrate 10 until after the covercoat layer 12 isdried. Depending on the process equipment and desired process sequence,the adherent liner 22 may be removed prior to the covercoat layer 12being exposed or after the covercoat layer 12 is exposed.

The adherent liner precludes the covercoat material from flowing duringthe drying step such that the lead portion 15 of the covercoat layer 12has a controlled and uniform thickness. The adherent liner 22 alsoenables the covercoat layer 12 to be photoimaged in a manner thatproduces a covercoat over the leads (lead portion) 15 having acontrolled offset distance 35, FIG. 5. The offset distance 35 is definedto be the distance from the adjacent respective edge of the basesubstrate 10 to the far edge of the lead portion 15 of the covercoat.

An embodiment of a printed circuit 30 is illustrated in FIGS. 4 and 5.One or more conductors are formed on the base substrate 10. Eachconductor includes a trace 32 and may be formed to include a lead 34that extends from the interior edge 19, the exterior edge 17 or bothedges of the base substrate 10. As shown in FIG. 5, the lead portion 15of the covercoat layer 12 is formed to cover at least a portion of thetop surface 40 and the side surfaces 42 (only one side surface shown) ofeach lead 34.

Another embodiment of a printed circuit 30 is illustrated in FIGS. 6 and7. The lead portion 15 of the covercoat layer 12 encapsulates a portionof each lead 34. The lead portion of the covercoat layer 12 alsoencapsulates an edge interface 44, FIG. 7, established at theintersection of each lead 34 and the respective edge of the basesubstrate 10. Encapsulating the edge interface 44 and the portion ofeach lead 34 formed adjacent to the edge interface 44 reduces thepotential for failures associated with corrosion at the edge interface44.

FIG. 8 illustrates yet another embodiment of a printed circuit whereinthe lead portion 15 of the covercoat layer 12 is completely detachedfrom the trace portion 13. The covercoat layer 12 is formed such thatthe lead portion 15 is carried by the leads 34 and is spaced apart fromthe trace portion 13 by a controlled and uniform offset distance 35. Thelead portion 15 of the covercoat layer 12 may be formed to fully orpartially encapsulate the adjacent portion of one or more leads 34.

In another embodiment, a substrate includes a base substrate having anopening formed therethrough. The opening defines an interior edge. Aphotoimageable covercoat layer is formed on the base substrate. Thephotoimageable covercoat layer includes a lead portion formed in theopening. The lead portion has a controlled offset distance andcontrolled and substantially uniform thickness.

A further embodiment provides a method of forming a covercoat layer on abase substrate. The method includes the steps of providing a basesubstrate having opposing sides; laminating an adherent liner to a sideof the base substrate; forming a photoimageable covercoat layer having alead portion formed on the adherent liner; and photoimaging the leadportion of the photoimageable covercoat layer.

As it can be seen, the embodiments presented herein provide severaladvantages, including the following advantages. The edge interface andthe surfaces of a trace adjacent the edge interface can be encapsulatedby the covercoat layer. The covercoat layer can be formed to include alead portion. The lead portion of the covercoat layer may be attached tothe trace portion or completely detached from an edge of the basesubstrate by a controlled offset distance. In the preferred embodiment,the thickness of the lead portion of the covercoat layer is alsocontrolled as desired, and is substantially uniform, although nonuniformthickness can also be made by this controlled method where desired. Thecovercoat layer can be patterned to include one or more photoimagedfeatures, and the method of manufacturing the embodiments presentedherein is economical to implement.

EXAMPLE 1

A flexible circuit having a polyimide base substrate with an openingformed through the base substrate was provided. The opening defines aninterior edge. The flexible circuit has a plurality of conductive tracesformed on a first side of the base substrate. At least a portion of theconductive traces includes a lead extending from the interior edge ofthe base substrate. An adherent liner having a polyester backing with ahighly crosslinked acrylate adhesive was laminated to a second side ofthe base substrate. The liner and the opening define a cavity. A layerof liquid photoimageable covercoat resin sold by Nippon Polytech (StockNo. NPR80/431) was coated onto the first side of the base substrate in amanner in which the covercoat substantially fills the cavity. Thecovercoat layer was then dried in an air convection oven atapproximately 80° C. for approximately 30 minutes. Following the dryingstep, the tape was removed from the base substrate. A patternedphotomask was positioned adjacent to the covercoat layer and thecovercoat layer was then exposed to a UV light source from the firstside of the base substrate. The UV light source was set to provideapproximately 500 mj/cm². The UV dosage was specified to providesufficient energy to cure the covercoat to a depth approximately equalto the thickness of the conductive traces. The covercoat was thendeveloped in an alkali solution to remove the appropriate areas of thecovercoat layer. The circuit was then baked at approximately 150° C. forapproximately 30 minutes to complete the curing of the covercoat layer.The resulting printed circuit resembles the embodiment illustrated inFIG. 5.

EXAMPLE 2

A circuit was formed using the same method as in Example 1, except thatthe covercoat was exposed to UV light from both sides of the basesubstrate. The two-sided exposure process provides a circuit wherein thetop, bottom and sides of the leads are encapsulated with covercoat. Theresulting printed circuit resembles the embodiment illustrated in FIG.7.

A wide range of modification, change and substitution is contemplated inthe foregoing disclosure and in some instances, some features of theembodiments may be employed without a corresponding use of otherfeatures. Accordingly, it is appropriate that the appended claims beconstrued broadly.

What is claimed is:
 1. A covercoated base substrate, comprising: a basesubstrate having an opening formed therethrough, the opening defining aninterior edge; and a photoimageable covercoat layer having a leadportion extending beyond the interior edge into the opening, thecovercoat layer having a controlled offset distance.
 2. The basesubstrate of claim 1 wherein said base substrate includes at least oneconductor, said conductor including a trace portion and a lead portion,said lead also extending into said opening.
 3. The base substrate ofclaim 2 wherein said covercoat covers at least one lead.
 4. The basesubstrate of claim 1 wherein said lead portion has a controlledthickness.
 5. The base substrate of claim 1 wherein said controlledthickness is substantially uniform.
 6. The base substrate of claim 1wherein the lead portion of the photoimageable covercoat layer includesa three-dimensional photoimaged feature formed therein.
 7. The basesubstrate of claim 6 wherein the photoimaged feature includes a channelhaving a depth of less than the controlled thickness of the covercoat.8. The base substrate of claim 6 wherein the photoimaged featureincludes a passage completely through said covercoat.